The present invention relates generally to semiconductor manufacturing processes, and specifically to methods and apparatus for cleaning and handling of lithographic masks used in producing semiconductor devices.
As the trend continues to reduce the size of semiconductor devices, optical lithography using conventional transmission masks, such as chrome on glass (COG) or phase shift (PSM) masks, will no longer suffice as a viable technique for printing advanced devices on semiconductor wafers. Transmission lithography has been extended to ever shorter wavelengths, down to 157 nm in the far ultraviolet (UV), in order to reduce the size of device features. However, the still shorter wavelengths necessary for printing even smaller device structures are readily absorbed in transmission materials. Alternative technological candidates to replace optical lithography include: electron projection lithography (EPL) and an all-reflective technology called extreme ultra-violet lithography (EUVL).
Virtually all masks used in production today employ a pellicle to protect the mask surface from particulate contamination. The pellicle is a relatively inexpensive, thin, transparent, flexible sheet, which is stretched above and not touching the surface of the mask. Pellicles provide a functional and economic solution to particulate contamination by mechanically separating particles from the mask surface. The mask is transported and used for lithographic exposure with the pellicle in place. When a mask is used for exposure, with the pellicle in position above the mask, only the details of the mask""s focal plane itself are printed. Particulate material located on the pellicle surface is maintained outside of the focal plane of projection. As a result, particulate material is not printed. When the pellicle eventually becomes damaged or too dirty to use, the mask is removed to a workshop, and the pellicle is replaced.
A suitable pellicle material and structure have yet to be defined for 157 nm technology. The options to date include using either no pellicle or a very expensive hard pellicle. An inexpensive soft pellicle that is capable of withstanding multiple exposures to 157 nm light has yet to be developed. It appears, therefore, that masks for lithography at 157 nm and for shorter wavelengths must be used without the protection of a pellicle. If a no-pellicle option is chosen, the masks must be cleaned frequently, and the cleaning technique must be suitable for multiple cleaning cycles without inducing any significant damage to sensitive mask films. Most contaminants absorb radiation at short wavelengths, and it is therefore imperative that the mask surface be completely free of any contamination that may absorb radiation.
Not only must mask particle contamination removal efficiency be increased, but the minimum particle size to be removed must also decrease. For example, in EUV lithography, masks must be cleaned to remove particles as small as 70 nm, since particles of this size are already printable at EUV lithography wavelengths. Conventional cleaning technologies such as sulfuric-peroxide mixture (SPM) and standard cleans (SC-1 and SC-2) do not fulfill all of the previously mentioned contamination removal criteria. If these conventional cleaning procedures must be applied to the mask repeatedly (due to the absence of a mask pellicle), they are likely to cause rapid degradation of delicate mask film layers.
Various methods are known in the art for stripping and cleaning foreign matter from the surfaces of semiconductor wafers and masks, while avoiding damage to the surface itself. For example, U.S. Pat. No. 4,980,536, whose disclosure is incorporated herein by reference, describes a method and apparatus for removal of particles from solid-state surfaces by laser bombardment. U.S. Pat. Nos. 5,099,557 and 5,024,968, whose disclosures are also incorporated herein by reference, describe methods and apparatus for removing surface contaminants from a substrate by high-energy irradiation. The substrate is irradiated by a laser with sufficient energy to release the particles, while an inert gas flows across the wafer surface to carry away the released particles.
U.S. Pat. No. 4,987,286, whose disclosure is likewise incorporated herein by reference, describes a method and apparatus for removing minute particles (as small as submicron) from a surface to which they are adhered. An energy transfer medium, typically a fluid, is interposed between each particle to be removed and the surface. The medium is irradiated with laser energy and absorbs sufficient energy to cause explosive evaporation, thereby dislodging the particles.
Various methods are known in the art for discriminating and localizing defects on substrates. U.S. Pat. Nos. 5,264,912 and 4,628,531, whose disclosures are incorporated herein by reference are examples. Foreign particles are one type of defects that can be detected using these methods.
U.S. Pat. No. 5,023,424, whose disclosure is incorporated herein by reference, describes a method and apparatus using laser-induced shock waves to dislodge particles from a wafer surface. A particle detector is used to locate the positions of particles on the wafer surface. A laser beam is then focused at a point above the wafer surface near the position of each of the particles, in order to produce gas-borne shock waves with peak pressure gradients sufficient to dislodge and remove the particles. It is noted that the particles must be dislodged by the shock wave, rather than vaporized due to absorption of the laser radiation. U.S. Pat. No. 5,023,424 further notes that immersion of the surface in a liquid (as in the above-mentioned U.S. Pat. No. 4,987,286, for example) is unsuitable for use in removing small numbers of microscopic particles.
It is an object of some aspects of the present invention to provide improved methods and apparatus for removal of microscopic particles from lithographic masks used in semiconductor device production. In the context of the present patent application and in the claims, the word xe2x80x9cparticlexe2x80x9d is used broadly to refer to any contaminant or other foreign substance that must be removed from the mask surface.
In embodiments of the present invention, a lithography tool, for use in producing semiconductor devices, comprises one or more lithography stations, together with a mask cleaning station. The lithography and mask cleaning stations are contained in a common enclosure, and a robot is preferably used to transfer the masks between the cleaning and lithography stations in order to isolate the mask and the stations from ambient air and from human contact. This arrangement is particularly advantageous in dealing with masks without pellicles, since it allows particles to be removed frequently from the masks, in the production environment, without removing the masks to a separate mask shop. This arrangement facilitates the higher level of mask cleanliness that is required for far UV and EUV lithography.
Preferably, the lithography tool also comprises an inspection station, which checks each mask before or after use to verify that the mask is still clean and, if not, to determine the locations of any contaminant particles on the mask. If the inspection station finds the mask to be contaminated, the robot passes the mask to the cleaning station. Based on coordinates of the particles determined by the inspection station, the cleaning station applies a local cleaning process to remove the particles. Preferably, the local cleaning process involves wetting the particle location with a suitable fluid, and then irradiating the location with laser radiation, most preferably UV laser radiation. This cleaning approach gives optimal removal of contaminant particles, without affecting in any way the remainder of the mask.
Alternatively, various other local cleaning methods may be used in conjunction with the inspection station. Examples of such methods include localized plasma application; local application of pressurized gas or vacuum; and local application of carbon dioxide dry ice (or xe2x80x9csnow cleaningxe2x80x9d). In addition, chemical cleaners in liquid and/or vapor state may be locally dispensed at the particle coordinates. In any case, when local cleaning is used, degradation of the mask due to frequent cleaning is minimized, and the useful life of the mask is thus lengthened.
There is therefore provided, in accordance with an embodiment of the present invention, apparatus for semiconductor device fabrication, including:
at least one lithography station, which is adapted to project a pattern of radiation from a mask onto a semiconductor wafer;
a mask cleaning station, which is adapted to receive the mask from the at least one lithography station, to clean the mask so as to remove a contaminant therefrom, so that the cleaned mask may be transferred to the at least one lithography station;
a robot, which is adapted to convey the mask between the at least one lithography station and the mask cleaning station; and
an enclosure, containing the at least one lithography station, the mask cleaning station and the robot, so that the mask is conveyed between the at least one lithography station and the mask cleaning station without human contact and without exposure to ambient air.
Preferably, the apparatus includes a mask storage station, contained at least partly within the enclosure, and the mask storage station is adapted to store the mask, and the robot is adapted to convey the mask between the mask storage station and the cleaning and lithography stations. Further preferably, the at least one lithography station includes a radiation source for generating the radiation that is projected onto the semiconductor wafer, and the radiation has a wavelength that is less than 160 nm.
In one embodiment, the at least one lithography station is adapted to project the pattern of radiation from the mask in the absence of a pellicle covering the mask. The at least one lithography station may include a plurality of exposure tools, commonly contained within the enclosure and served by the mask cleaning station.
Preferably, the apparatus includes an inspection station, which is adapted to determine position coordinates of the contaminant on the mask, and to convey the coordinates to the cleaning station, which is adapted to clean the mask locally at a location indicated by the coordinates. Most preferably, the inspection station is contained within the enclosure. In one embodiment, the cleaning station includes a vacuum source, which is adapted to apply suction at the location indicated by the coordinates. The cleaning station may additionally include an inlet port, and may be adapted to inject a fluid medium through the inlet port so that the fluid medium is deposited on the mask at the location indicated by the coordinates, prior to applying the suction thereto.
In another embodiment, the cleaning station includes an inlet port, and is adapted to inject a pressurized cleaning medium through the inlet port so that the cleaning medium impinges on the mask at the location indicated by the coordinates.
Preferably, the cleaning station includes a radiation source, which is adapted to generate a beam of electromagnetic energy, and the cleaning station is adapted to controllably direct the beam of electromagnetic energy toward a location of the contaminant on the mask, causing the contaminant to be dislodged from the mask substantially without damage to the surface itself. Preferably, the cleaning station includes a gas inlet, and the cleaning station is adapted to inject an energy transfer medium through the gas inlet so that the medium is deposited on the mask at the location of the contaminant. Most preferably, the medium absorbs at least a portion of the electromagnetic energy incident on the mask, causing local evaporation of the medium, which dislodges the contaminant. Preferably, the electromagnetic energy includes ultraviolet laser energy. Further preferably, the energy transfer medium includes a carrier gas with a condensable vapor. Typically, the condensable vapor is water.
Further preferably, the cleaning station is adapted to receive input position coordinates of the location of the contaminant on the mask, and to direct the medium and the beam so that the medium and beam are incident on the mask at the location indicated by the position coordinates. Most preferably, the cleaning station further includes an inspection station, which is adapted to determine the input position coordinates and to convey the coordinates to the cleaning station.
There is also provided, in accordance with an embodiment of the present invention, a method for semiconductor device fabrication, including the steps of:
enclosing at least one lithography station and a mask cleaning station in an enclosure, so that a mask may be conveyed between the at least one lithography station and the mask cleaning station without human contact and without exposure to ambient air;
cleaning the mask in the mask cleaning station so as to remove a contaminant therefrom;
conveying the mask within the enclosure from the mask cleaning station to the at least one lithography station; and
projecting a pattern of radiation from the mask onto a semiconductor wafer in the at least one lithography station.
Preferably, projecting the pattern of radiation includes generating the radiation at a wavelength that is less than 160 nm. Further preferably, projecting the pattern of radiation includes projecting the pattern of radiation from the mask in the absence of a pellicle covering the mask.
Preferably, the method includes storing the mask in a mask storage station, which is at least partially contained in the enclosure, and conveying the mask includes transferring the mask between the at least one lithography station, the mask cleaning station, and the mask storage station. Further preferably, cleaning the mask includes determining position coordinates of the contaminant on the mask, and cleaning the mask locally at a location indicated by the coordinates. Most preferably, determining the position coordinates includes enclosing an inspection station within the enclosure, and inspecting the mask using the inspection station.
Preferably, cleaning the mask includes controllably directing a beam of electromagnetic energy toward a location of the contaminant on the mask, so as to cause the contaminant to be dislodged from the mask substantially without damage to the mask itself. Further preferably, cleaning the mask also includes controllably applying an energy transfer medium at the location of the contaminant on the surface, wherein the beam of electromagnetic energy causes local evaporation of the medium, thereby dislodging the contaminant.
Preferably, the electromagnetic energy includes ultraviolet laser energy.
Preferably, controllably directing the electromagnetic energy includes receiving input position coordinates of the location of the contaminant on the mask, and directing the beam so that the beam is incident on the mask at the location indicated by the position coordinates.